The representative prior art for refrigeration defrost control is disclosed in the following U.S. Patents: U.S. Pat. Nos. 3,899,895 (Blanton et al); 3,839,878 (Tilmanis); 3,826,103 (Grover); 3,759,049 (Bell et al); 3,726,105 (Auracher); 3,626,707 (Bauknecht et al); 3,525,222 (Schuller); 3,518,841 (West, Jr.); 3,492,832 (Davis et al); 3,436,929 (Harbour); 3,373,575 (Nelson); 3,228,204 (Matthies); 3,203,195 (Armentrout); 3,174,297 (Kuhn et al); 3,138,006 (Moorman et al); 3,134,238 (Matthies); 3,105,364 (O'Connell); 3,055,188 (Syfert); 2,949,017 (Swanson); 2,907,180 (Mann); 2,866,323 (Candor); and 2,765,630 (Shaw).
The patent to Moorman, et al (U.S. Pat. No. 3,138,006) discloses a defrost control arrangement for a two compartment refrigerator, one compartment generally operating below-freezing and the other operating at above-freezing temperatures. Warmer humid air is drawn from the above-freezing compartment to the below-freezing compartment and directed over the evaporator to cool and remove moisture from the warm humid air. The cooled air is directed downwardly by a fan to the above-freezing compartment through a passageway, the airflow through which is controlled by a thermostatically controlled air valve. The defrost control includes a snap-acting, double-throw, bimetal, thermostatic switch that is mounted upon the edges of the fins of the evaporator at the point where the warm humid air from the above-freezing compartment enters the evaporator chamber and is responsive to the temperature of the warmer air flowing from the above-freezing compartment and the temperature of the evaporator surface.
The switch has first and second contacts which are alternately energized upon a rise in temperature to about 55.degree. F. and to a fall in temperature to about 28.degree. F. when the switch has frost formed about its outer surface. The refrigerating system is connected to the energy supply upon the rise in temperature of the switch to 55.degree. F. and remains connected until the temperature of the switch falls to 28.degree. F. and is frosted. When the temperature falls to 28.degree. F. the defrost system is connected to the energy supply for defrosting the evaporator. When the thermostat switch is free of frost, it is warmed by the relatively warm air from the above-freezing compartment, thus preventing the thermostat from falling to the low defrost temperature. After frost accumulates on the thermostat and the evaporator, the rate of air flow is reduced and the thermostat is shielded from the warmer air from the above-freezing compartment by the frost covering on the thermostat. This shielding action by the frost lowers the temperature of the thermostat. To prevent unnecessary defrosting by the thermostat because of temperature variations, a small electric heater is provided that is in heat transfer with the thermostat and normally energized to compensate for such temperature variations.
When the above-freezing compartment rises to an abnormally high temperature, the air valve will move to an abnormally wide-open position and opens a switch contact in series with the thermostat heater, permitting the thermostat to cool if the thermostat has sufficiently frosted over to lower the temperature below 28.degree. F. The thermostat will snap to the defrost position and energizes a defrost heater to melt the frost from the evaporator. The thermostat heater is also controlled by a temperature responsive resistor responsive to ambient temperatures.
The above prior art defrost control utilizes as a defrost initiation device the cooperative temperature responsive activity of the air-valve (responsive to temperature in the above-freezing compartment) and the frosting of the thermostatic switch in physical contact with the evaporator. The thermostat heater power is controlled by temperature responsive resistances responsive to ambient temperatures to vary the need for defrost and the defrost period. Such control results in a further cooperative temperature response to initiate defrost action. One major disadvantage is that the thermostat must be placed in a location where it can be sufficiently frosted to shield the thermostat from the high-temperature compartment air for initiating the defrost cycle. The ideal location for a defrost cycle termination thermostat would be in an area where the evaporator coil compartment is coldest, which would dictate a location other than that for the placement of the thermostat for initiating the defrost cycle. However, since the thermostatic switch serves a dual purpose (termination and initiation) the actual location for responding to the high temperature compartment air is not the desirable loation for terminating the defrost. Because of the extremely specialized nature of the above control, it never met with commercial acceptance in the marketplace.
The other patents disclose defrost controls that operate in response to differentials in temperature of the evaporator coil and the refrigerated space; in response to clock timers and humidity sensors; in response to heating the evaporator coil using heated refrigerant; or in response to other mechanical switch devices.